1. Field of the Invention
The present invention relates to improvements in the art of photo-keratometry and pachymetry and, more particularly, to the use of television techniques to ascertain the contour and thickness of the cornea. More specifically, the present invention relates to improvements in the art of photogrammetry where the reflection of a placido or other illuminated target by the surface of the contact cornea is analyzed to determine the surface contour and analysis of steroscopic images of point type Tyndall images for establishing thickness of the cornea. A pachymeter is an instrument for measuring the thickness of the cornea, commonly by ultrasound or optical devices. A keratometer is an instrument for determining the shape of the corneal surface which often uses a placido or other illuminated target which is reflected from the surface to be characterized for surface contour. The present invention measures both the keratorefractive surface shape by a modification of Placido's device coupled with a photo triangulation Tyndall image analysis method for determining the corneal thickness.
2. Brief Description of Related Art
Placido's method as applied to corneal surface measurements dates from the middle of the last century through the early thirties when the Zeiss optical company of Germany introduced a "Photo Keratoscope" designed by Amsler of Lausanne Switzerland. In general, the art has required the image reflected by the eye to be photographed and the image on the film measured in a second step to derive the data from which the contour map is generated. Initial development of keratometry came from Helmholtz in 1854, Placido published an article in 1880 "A Novo instrumiento par analyze immediate das irregularidades de curvature de cornea" Periodico Ophthalmol Practica 1880;6:44-49. Gullstrand expanded the basis for the present invention in his "Photographisch-Ophhthalmometrische und klinische Untersuchungen uber die Hornhautrefraktion"(from Ludlam translation Am.J. Optom. 1966;43;143-198). In 1896, Gullstrand disclosed the foundation for the current photographic based technology but his apparatus had no provision for automatic data assessment and was limited to a four millimeter diameter zone. As a result, multiple exposures and calculations were necessary to map the corneal surface.
A more recent attachment for the ubiquitous Bausch and Lomb keratometer is the Topogometer produced by Soper Brothers of Houston, Tex. This device provides a series of fixation points for the patent to look at which permits largest areas of the cornea to be described. This method is inaccurate because the calibration assumes spherical surfaces or aspheric surfaces centered on the image center of the instrument. Because the corneal surface is aspheric even in the normal cornea, this multiple fixation technique produces errors which are a function of point of gaze but which are undefinable.
U.S. Pat. No. 3,797,821 discloses the use of a camera to record the placido reflex from a patient's eye. From this photograph, the radius of surface of curvature of the cornea is determined at several points is calculated using a complex computer system. The use of a ground glass focusing screen with the small aperture of the optical system and large linear magnification makes use difficult and requires a darkened room for operation. Additionally, the method utilizes a separate video/computer analysis step which further degrades accuracy, speed and increases cost of operation. More recently, U.S. Patents, such as U.S. Pat. No. 5,841,511 to D'Aousa, et al, have addressed the problem of the inherent ambiguity of the concentric circle form of the Placido while U.S. Pat. No. 5,847,804 to Sarver, et al attempts to define the corneal apex through the use of an additional camera which purports to locate the apex and limbus. In fact, these do provide some enhancement of the basic technique from the last century, but they fall short of the goal of exact definition of corneal surface shape and cannot provide thickness information which is imperative for many surgical procedures.
U.S. Pat. No. 4,440,477 discloses a method and device for measuring the corneal surface, comprising a slit lamp for illuminating the corneal surface, a camera for recording the reflection from the corneal surface, and a processor to calculate the image distance and the radius of curvature of the eye. the operation of the processor is not detailed in U.S. Pat. No. 4,440,477. Additionally the prior art devices do not work well in the presence of reflections from objects or lights int he room and do not provide rapid, accurate measurements such as are required fro modern contact lens fitting. Consequently, the systems are costly, complex, slow and difficult to change if required.
The traditional approach to photogrammetry has bene very software intensive and, thus, quite costly. The most common technique is to convert the entire television image to digital form prior to sorting, calculating by matrix algebra techniques, and display. The digitized image must occupy only a portion of the available memory in any computer system if there is to be the capacity to act upon the image information. More recent technique is described in my U.S. Pat. Nos. 5,110,200, 4,412,965, as well as Gersten's work such as disclosed in U.S. Pat. No. 5,384,608. The present invention addresses some of the problems found with the prior art.
Placido based instruments are the most common of the keratometric instruments of the prior art. Inherent in the Placido system are several assumptions which have a bearing upon the accuracy of the measurements. Among these assumptions are:
1. The corneal contour can be defined accurately from reflected concentric circles on the tear film covering the cornea. PA1 2. The Placido image plane and location of the corneal apex are know. PA1 3. The measurement of anterior surface curvature in dioptric terms is an accurate measure of the focusing power of the eye. PA1 4. Valid data can be obtained from very small reflections near the center of the reflection of Placido's disc.
When photographs of the reflections of Placido's disc are made the contour of the cornea is such that only the middle zone can be measured since the more peripheral zone is sloped in such a way as to prevent the reflected image to be seen to be a centrally located camera. The central zone is not resolved because the size of the reflection approaches the inherent resolution of the camera employed and the error of determination increases to infinity at the center of the image being analyzed. Measurements of the ring reflections by back tracing rays assumes that the exact point of origin of a ray from a continuous line can be made which is clearly not possible. Because there is no possible measurement at center, even if the reflection of a fixation target is assumed to define the corneal apex, the exact location of the image plane behind the corneal surface and the true apex are unknown. However, both of these must be known for accurate measurement results. Back tracking rays can define the tangent slope of points on the cornea to a fair degree of accuracy, but only if the initial point used in the constructing the surface model is known which is not possible in simple Placido based designs. A keratometer based on Placido's method can only measure anterior surface curvature in terms of radius of curvature at selected points. Because the posterior surface of the cornea in conjunction with the aqueous film constitutes a negative lens, the effective dioptric power is the algebraic sum of the two "lenses". For the keratometric measurement to be strictly accurate, the corneal thickness must be constant, known and perfectly concentric on both the anterior and posterior surfaces. For example, the common Bausch. & Lomb keratometer used in most clinical settings uses a biased value of index of refraction in the conversion from radius of curvature to dioptric form to compensate for this problem. My recent U.S. Pat. Nos. 5,885,767 and 5,735,283 describe apparatus and method for determining the needed information but have an inherent limitation caused by distortion of the projected image points by both surface slope and corneal thickness at the point of reflection.